Modern computing devices often attempt to achieve a balance between portability and functionality. A tension can exist between having a display that provides for a rich display of information on a single surface, which suggests a relatively large form factor of the device to accommodate a relatively large display, and a device that is small enough to be easily carried and accessed by a user, which suggests a relatively small form factor of the device.
A potential solution to address this dilemma is to use a foldable flexible display in the computing device, so that in the display's folded configuration, the computing device has a relatively small form factor, and in the display's unfolded configuration, the computing device can have a relatively large display. To keep the form factor of the computing device small and slim, it is desirable to have relatively thin displays. However, folding a relatively thin display can result in small radius bends at the fold in the display, which may be detrimental to sensitive components of the display, for example, thin film transistors (TFTs), organic light-emitting diodes (OLEDs), thin-film encapsulation (TFE) and the like. In addition, thin displays can be relatively fragile and in need of protection against breakage from impacts to the front surface of the device.
It can be difficult to create foldable top-emitting plastic OLED displays that have a small folding radius in both directions (i.e., having two surfaces of the display fold both towards each other and away from each other) and that can survive many fold-unfold cycles. In particular, creating sturdy, durable Z-fold displays (i.e., displays with both inward and outward folds) is greatly complicated by the fragility of the thin-film layers in the display stack.
One approach is to building the stack of layers for a functional display is to use optically clear adhesive (OCA) to join different functional layers of the stack. For example, a display stack may include from the following layers:                1. Backplate layer        2. Adhesive layer        3. Display layer (including polyimide substrate with barrier, TFT, OLED, and encapsulation layers)        4. OCA layer        5. Touch sensitive layer (typically a multi-layer film stack)        6. OCA layer        7. Polarization layer (including a circular polarizer)        8. OCA layer        9. Cover Window (CW) layer (user-facing cover window film)        
In some implementations, the polarization layer and the touch sensitive layer may be reversed, combined or eliminated. A common development direction involves building touch functionality directly on top of the display layer. This reduces the thickness of the stack of the most fragile layers and also simplifies electrical connection to the touch layer. In such implementations, the stack may include the following layers:                1. Backplate layer        2. Adhesive layer        3. Display-Touch layer        4. OCA layer        5. Polarization layer (including a circular polarizer)        6. OCA layer        7. CW layer (user-facing cover window film)        
In another implementation, the cover window layer and the polarization layer can be combined, so that the stack includes the following layers:                1. Backplate layer        2. Adhesive layer        3. Display-Touch layer        4. OCA layer        5. POL-CW layer        
The Display-Touch layer often is manufactured in a very expensive, highly-automated OLED factory using a highly optimized recipe that cannot easily be altered to meet customer customer-specific requirements. The backplate and polarization/cover window layers may be customer-specific and are typically added in a less expensive factory setting after the display exits the OLED line. However, these customer-specific backplate layer and polarization/cover window layers may cause the neutral plane of the device to shift away from the display-touch layer, which may be detrimental to the in-system folding cycle life of the entire display.
Thus, foldable display devices in which the neutral plane of the device is in, or close to, the most fragile display layers, are desirable.